In the interest of improving cancer treatment, considerable attention has been placed on the modification of radiation damage. The major goal of this project is to define and understand those aspects of tumor physiology, including cellular and molecular processes that ultimately define the very nature of a tumor such that a particular dose of ionizing radiation, when used will be more effective. One means to that end is to investigate the interaction of ionizing radiation with a variety of chemotherapy agents to assess if tumors can be made more sensitive. Our current focus is on ecteinascidin 743 and halifuginone. Ecteinascidin exhibits anti-tumor activity presumably by alkylation of guanine N2 resulting in inhibition of DNA repair enzymes, transcriptional effects, and inhibition of topoisomerases. We have observed significant enhancement of the radiation response when human colon carcinoma cells were pretreated with relatively non-toxic concentrations of ecteinascidin. in vivo studies are presently underway. Halifuginone, an inhibitor of TGF beta, also radiosensitizes several human tumor cell lines. This agent is of particular interest in that halifuginone has been shown to protect against radiation-induced late effects in normal tissues. Another major thrust of this project is to develop functional imaging techniques to better characterize factors important in the tumor microenvironment that may prevent or diminish agents from impacting radiation response. It is well established that hypoxia is a major determinant of radiation sensitivity. Therefore, we are using several murine tumor models to study tumor hypoxia. Our approach is to use current invasive techniques and extend that information to non-invasive methods that are under development, such that patient tumor treatment profiles may optimized on an individual basis. Using novel magnetic resonance imaging equipment developed in the Radiation Biology Branch we have recently: a) validated non-invasive Overhauser magnetic resonance imaging of tumor oxygen levels with "gold standard" oxygen electrode measurements, b) demonstrated using electron paramagnetic resonance imaging that the in vivo reduction rate of the nitroxide to the hydroxylamine can yield a "redox map" of various tissues including tumor, and c) demonstrated the concept of "oxidative imaging" using a double ester nitroxide/hydroxylamine precursor molecule. This technique can report on tissue oxidation as a result of radiation treatment or other oxidative processes. Collectively, these non-invasive functional imaging approaches should enhance our ability to better understand the tumor microenvironment and develop strategies to effectively attack potential barriers that currently limit the effectiveness of cancer treatment modalities.